It is known that an electronic apparatus such as a switching power supply carries noise that may have a wide range of frequencies from a frequency as low as 100 kHz to a frequency as high as several hundred MHz. Noise emerging from an electronic apparatus is sent to another electronic apparatus through an alternate current power line, and may affect the other electronic apparatus. Therefore, in many countries, various restrictions are placed on conducted noise, that is, noise emerging from an electronic apparatus and emitted outside through an alternate current power line. According to the restrictions on conducted noise imposed in a great part of such countries, the frequency range to be restricted ranges from 150 kHz or 450 kHz to 30 MHz. The noise at a frequency of 30 MHz or higher applies to a restriction on emission noise.
FIG. 21 and FIG. 22 illustrate examples of frequency characteristics of conducted noise of a switching power supply wherein no measure is taken for suppressing noise. FIG. 21 shows the characteristic in a frequency range of 0 to 1 MHz. FIG. 22 shows the characteristic in a frequency range of 0 to 200 MHz. In FIG. 21 and FIG. 22 peak values of the conducted noise are shown. In FIG. 21 and FIG. 22 numeral 201 indicates a frequency range in which common mode noise exists. In FIG. 21 numeral 202 indicates a frequency range in which normal mode noise causes a problem. In FIG. 21 and FIG. 22 numeral 203 indicates an example of frequency range that applies to the restriction on conducted noise. In FIG. 21 numeral 204 indicates a permissible level of conducted noise according to the European Standard EN55022 corresponding to the standard of the International Special Committee on Radio Interference (CISPR). In FIG. 21 numeral 205 indicates a permissible level of conducted noise according to the Class B of the standard of the Federal Communications Commission (FCC). As shown in FIG. 21 and FIG. 22, the noise emerging from the switching power supply exists in a wide range of frequencies including a range of 150 kHz to 30 MHz that applies to the restriction on conducted noise and a range of 30 MHz and higher that applies to the restriction on radiated noise.
In FIG. 21 a frequency of approximately 75 kHz at which a great peak of conducted noise arises is a switching frequency of the switching power supply. As shown in FIG. 21, it is noted that a plurality of harmonics pertaining to the switching frequency creat a great noise.
To prevent the adverse effect of noise as described above, in prior art, a noise filter circuit as shown in FIG. 23 is often provided between the power line and the electronic apparatus that develops noise. The noise filter circuit of FIG. 23 will now be described. The noise filter circuit comprises: two terminals 101a and 101b connected to the power line; and two terminals 102a and 102b connected to the electronic apparatus as a noise source. The noise filter circuit further comprises: a capacitor 111 having an end connected to the terminal 101a and the other end connected to the terminal 101b; a common mode choke coil 112 provided between the terminals 101a and 101b and the terminals 102a and 102b; a capacitor 113 having an end connected to the terminal 102a and the other end grounded; a capacitor 114 having an end connected to the terminal 102b and the other end grounded; and a capacitor 115 having an end connected to the terminal 102a and the other end connected to the terminal 102b. The common mode choke coil 112 has one magnetic core 112a and two windings 112b and 112c wound around the core 112a. The winding 112b has an end connected to the terminal 101b and the other end connected to the terminal 102a. The winding 112c has an end connected to the terminal 101b and the other end connected to the terminal 102b. The windings 112b and 112c are wound around the core 112a in such directions that, when magnetic fluxes are induced in the core 112a by currents flowing through the windings 112b and 112c when a normal mode current is fed to the windings 112b and 112c, these fluxes are cancelled out by each other.
Among the components of the noise filter circuit of FIG. 23, the common mode choke coil 112 and the capacitors 113 and 114 have a function of reducing common mode noise. Typically, the combination of the capacitors 113 and 114 is called a Y capacitor. The capacitors 111 and 115 have a function of reducing normal mode noise. Each of the capacitors 111 and 115 is typically called an X capacitor or an across-the-line capacitor.
Problems that the noise filter circuit of FIG. 23 has will now be described. The problem relating to the measure taken against common mode noise of the noise filter circuit will be described first. In many countries the leakage current flowing through the capacitors 113 and 114 of FIG. 23 is limited to a specific standard value or below so as to assure safety, that is, to prevent shock hazards. The leakage current is proportional to the capacitance of the capacitors 113 and 114. Therefore, the capacitance of the capacitors 113 and 114 is limited to a specific value or below. The capacitance of the capacitors 113 and 114 is typically limited to several thousand picofarads (pF) or below although the standard value of leakage current depends on the circumstances and the supply voltage of each country.
FIG. 24 shows an example of frequency characteristic of impedance of the Y capacitor including the capacitors 113 and 114. FIG. 24 shows the characteristics each obtained when the capacitance of each of the capacitors 113 and 114 is 330 pF, 680 pF, 1000 pF, 2200 pF, 4700 pF or 10000 pF.
In the example of frequency characteristic of conducted noise of the switching power supply of FIG. 21, the noise level is high in a frequency range of 500 kHz and lower. However, as shown in FIG. 24, the impedance of the Y capacitor is high in a frequency range of 500 kHz and lower, and the Y capacitor does not make a great contribution to a reduction in common mode noise in this frequency range. Therefore, the noise filter circuit of FIG. 21 has a problem that it is not capable of sufficiently reducing common mode noise in a frequency range of 500 kHz and lower.
The problem relating to the measure taken against normal mode noise of the noise filter circuit of FIG. 23 will now be described. In the noise filter circuit of FIG. 23, there exists leakage inductance that results from leakage of magnetic flux in the common mode choke coil 112. FIG. 25 illustrates a circuit made up of the noise filter circuit of FIG. 23 to which imaginary coils having an inductance equal to the leakage inductance are added. The circuit of FIG. 25 is made up of the circuit of FIG. 23 in which the imaginary coil 116 is provided between the winding 112b of the common mode choke coil 112 and the terminal 102a, and the imaginary coil 117 is provided between the winding 112c of the common mode choke coil 112 and the terminal 102b. 
In the circuit of FIG. 25 the elements having a function of reducing normal mode noise are the capacitors 111 and 115 and the coils 116 and 117. These elements make up a π filter. However, the inductance of the coils 116 and 117, that is, the leakage inductance of the common mode choke coil 112 depends on the inductance of the coil 112 and the coupling coefficient between the windings 112b and 112c of the coil 112. Therefore, typically, it is difficult that the leakage inductance has a great value and it is difficult to design the circuit such that the leakage inductance has a specific value.
Typically, normal mode noise is problematic in a low frequency range of 1 MHz and lower. In such a frequency range the absolute value of impedance of the coil is expressed as 2 πfL where ‘f’ is a frequency. Therefore, a coil having a high inductance is required for increasing the absolute value of impedance of the coil and thereby sufficiently reducing normal mode noise in a low frequency range of 1 MHz and lower. Because of these reasons, the noise filter circuit of FIG. 23 has a problem that it is not capable of sufficiently reducing normal mode noise.
To overcome the foregoing problems, many of actual noise filter circuits have a configuration as shown in FIG. 26. The noise filter circuit of FIG. 26 comprises: the two terminals 101a and 101b connected to the power line; and the two terminals 102a and 102b connected to an electronic apparatus as a noise source. The noise filter circuit further comprises: a fuse 121 having an end connected to the terminal 101a; a capacitor 111 having an end connected to the other end of the fuse 121 and the other end connected to the terminal 101b; and the common mode choke coil 112 connected to the ends of the capacitor 111. The common mode choke coil 112 has the one magnetic core 112a and the two windings 112b and 112c wound around the core 112a. The winding 112b has an end connected to the other end of the fuse 121. The winding 112c has an end connected to the terminal 101b. 
The noise filter circuit of FIG. 26 further comprises: a capacitor 122 having an end connected to the other end of the winding 112b and the other end connected to the other end of the winding 112c; a capacitor 123 having an end connected to the other end of the winding 112b and the other end grounded; a capacitor 124 having an end connected to the other end of the winding 112c and the other end grounded; and a common mode choke coil 125 provided between the capacitors 123 and 124 and the terminals 102a and 102b. The common mode choke coil 125 has one magnetic core 125a and two windings 125b and 125c wound around the core 125a. The winding 125b has an end connected to the one of the ends of the capacitor 123 and has the other end connected to the terminal 102a. The winding 125c has an end connected to the one of the ends of the capacitor 124 and has the other end connected to the terminal 102b. 
The noise filter circuit of FIG. 26 substantially has a configuration in which the common mode choke coil 125 is added to the noise filter circuit of FIG. 23. According to the noise filter circuit of FIG. 26, the common mode choke coil 125 works for compensating for the insufficient function of reducing common mode noise and the insufficient function of reducing normal mode noise of the noise filter circuit of FIG. 23. However, the noise filter circuit of FIG. 26 has a problem that, since it has the two common mode choke coils 112 and 125, the circuit configuration is complicated and the noise filter circuit is increased in size.
In the noise filter circuit of FIG. 26, it is required to increase the leakage inductance of each of the common mode choke coils 112 and 125 so as to sufficiently reduce normal mode noise in a low frequency range of 1 MHz and lower. To achieve this, it is required to increase the number of turns of the coils 112 and 115. However, if the number of turns of the coils 112 and 115 is increased, there arises a problem that the interwinding stray capacitance is increased and the effect of reducing noise in a high frequency range is reduced. Alternatively, each of the coils 112 and 125 may have a structure in which the turns are divided into a plurality of portions by separators to reduce the interwinding stray capacitance of the coils 112 and 125. However, this raises the costs of the coils 112 and 125.